I have heard 3-6 weeks in advance for holiday parties, since everyone seems to get booked up so fast. Have also heard up to 8 weeks in advance for things right around the holidays, such as a child's birthday party when it falls close to the 25th.
We are having two parties coming up.
First - a holiday party (just friends, no family in immediate area) - I sent out invites 4 weeks in advance and am getting a good rsvp rate, only one person is already busy that night.
Second - my son's first birthday party - he was born on 12-26. This party will be out-of-state and mainly immediate family. I sent these invites out 8 weeks in advance, since it may interfere with family Christmas get-togethers. I have gotten less than 6 rsvp's! I don't know if I sent out the invites too early or if people feel like since they are family they don't actually need to rsvp. Even my FIL, who travels a lot, talked about the party but never said if they would be in town and able to come! I didn't know if it was polite to ask.

Someone once told me that if it was a holiday (they meant Christmas time frame) event, to try to get the invitations to them before just before Thanksgiving. Her reasoning was that a lot of families start making December plans over the Thanksgiving weekend.

Speaking US, of course. In Canada, that really WOULD be too early.